migrate the othello off hooked transformer

[MdSadiqMd]

Description

Migrate demos/Othello_GPT.ipynb from HookedTransformer(cfg) to TransformerBridge.boot_native(cfg), unblocking HookedTransformer deprecation at 4.0.

Motivation: The Othello demo is the public face of train-from-scratch and is in the active CI matrix. It instantiates HookedTransformer(cfg) directly, which will break at 4.0 when HT is deprecated.

Changes:

1. New convert_tl_checkpoint() utility (transformer_lens/conversion_utils/tl_checkpoint.py):

   • Converts TL-format state_dicts (keys like embed.W_E, blocks.0.attn.W_Q) to native bridge format (keys like tok_embed.weight, layers.0.attn.q.weight)
   • Handles attention weight reshaping: [n_heads, d_model, d_head] → [n_heads*d_head, d_model]
   • Handles MLP/unembed transposition

2. Updated demos/Othello_GPT.ipynb:

   • Changed imports to use TransformerBridgeConfig, TransformerBridge, convert_tl_checkpoint
   • Model creation now uses TransformerBridge.boot_native(cfg)
   • Checkpoint loading chains through convert_tl_checkpoint() before load_state_dict()
   • Both main path and LOAD_AND_CONVERT_CHECKPOINT path updated

3. Unit tests (tests/unit/conversion_utils/test_tl_checkpoint.py):

   • test_convert_tl_checkpoint_produces_matching_outputs: Verifies HT → bridge conversion produces identical outputs
   • test_convert_tl_checkpoint_key_mapping: Verifies all keys are correctly mapped
   • test_convert_tl_checkpoint_shape_transformations: Verifies tensor reshaping is correct

Verification:

• Notebook output matches expected (argmax identical to original)
• All 13 notebook cells pass --nbval-lax

Closes #1663

Type of change

• New feature (non-breaking change which adds functionality)

Screenshots

N/A

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

[jlarson4]

Great work @MdSadiqMd, thanks for your help! Merging now